In order to test the unified scheme for luminous radio galaxies and
quasars we observed 10 galaxy/quasar pairs from the 3CR catalogue with
ISOPHOT at infrared wavelengths between 5 and 180 μm. Each pair
was selected such that both the 178 MHz luminosity and the redshift
match as close as possible between the radio galaxy and the quasar in
order to minimize effects of cosmic evolution. 13 of the 20 sources
were detected in at least one waveband.
12 sources show clear evidence of a thermal bump at FIR
wavelength, while in the remaining 7 sources the upper limits
are still compatible with the presence of luminous dust emission.
In agreement with the predictions of the unified scheme, the quasars
and galaxies in our sample cannot be distinguished by their observed mid- and
far-infrared properties.
This is in contrast to the findings on the basis of the IRAS scans
which indicated that radio galaxies radiate significantly less mid- to
far-infrared emission than quasars. However, the IRAS samples are
dominated by low-redshift sources (), while our sample
contains several of the most luminous radio galaxies at redshift . The latter have already been suspected to contain a hidden
quasar for other reasons, e.g. an extended emission line region
aligned with the radio axis. From the ratio between FIR luminosity
emitted by dust and the radio power at 178 MHz, we conclude that the
radio galaxy/quasar unification might be perfectly valid for the most
luminous 3C sources at high redshift (). At lower redshifts
(), however, some of the lobe-dominated FRII radio galaxies
contain active nuclei which emit less UV-optical continuum than the
quasars of similar radio power. As this division is mainly a function
of redshift and less one of absolute radio power, we suggest that it
is caused by the evolution of the nuclear fueling rate with cosmic
epoch. In order to quantify the deviation from the purely
aspect-dependent unified scheme at low redshifts a larger fraction of
3C radio galaxies has to be observed at mid- to far-infrared
wavelengths with sensitivities which suffice to yield secure
detections rather than upper limits.

Based on observations with the Infrared Space Observatory
ISO, an ESA project with instruments funded by ESA Member States
(especially France, Germany, the Netherlands and the UK)
and with the participation of ISAS and NASA.

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